To me, the major theme of the story is the dangers of unrestricted scientific knowledge and man's inability to use that knowledge for good. This is based around the striking quote:

Perhaps, in the old days, they ate knowledge too fast.

Clearly the story paints a horrific dystopian picture of what could happen with the ever-daunting power man gains through science. Other themes that can be identified are the coming of age theme, truth and how we relate to it, and lastly, civilisation. What you need to do now is work on unpacking each of those themes and identify quotes that relate to each one to consider what the story has to say about each theme.